A recent paper published in Nature Geoscience has proposed the existence of an entirely new kind of volcanic eruption, which geologists are now calling a 'tangaroan' eruption. Rather than being either explosive or effusive, this newly documented type of eruption involves the slow release of magma that generates a kind of buoyant foam filled with tiny bubbles. Then, these globs of hardening molten lava rise to the ocean surface, where they become waterlogged and sink back down.

Floating chunks of volcanic rock are nothing new to scientists. When explosive underwater eruptions happen, gas bubbles called "vesicles" expand so quickly that they fragment the magma, causing it to cool and de-gas — thus creating a form of solidified pumice that's light enough to float on water.

And in fact, these rocks are sometimes found floating on the water in huge quantities. Just last year a mass of floating rocks were found off New Zealand — an expanse of molten rock that occupied a space the size of Belgium (about a 26,000 square kilometer stretch (10,039 square miles)).

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But this previously undocumented form of volcanic eruption is a bit different.

According to the researchers, a team consisting of Ian Wright and Melissa Rotella of the National Oceanography Centre, recent analysis on volcanic remnants indicated a new kind of process. Specifically, they documented the shape and density of bubbles in pumices generated by the Macauley volcano, an underwater caldera volcano in the southwest Pacific Ocean. They found large differences in the number and shape of bubbles in conventionally sized pumice — an indication that something different was going on geologically.

They figured that the bubble densities could not have formed from an explosive or effusive event, as the eruption was neither vigorous or gentle enough to produce the pumice. Instead, there had to be a kind of Goldilocks eruption — something in between these two well known processes.

In turn, the researchers are suggesting that, instead of exploding in the neck of the volcano, the formation and expansion of bubbles in the magma creates a kind of buoyant foam — which are called 'blebs.' This material eventually rises to the seafloor, where it assumes the form of a "molten pumice balloon" and floats up to the sea surface. But as it rises, the vesicles within the molten interior continue to expand as the water pressure gradually diminishes.

And it's this process that explains the unique bubble structures and densities seen in the samples; as a result, the tangaroan eruption is being thought of as an intermediate kind of volcanic eruption.